A. Foucaran

Anemometer with hot platinum thin film

The techniques of micromachining silicon are used for the manufacture of an anemometer with low electric consumption and great sensitivity. To reduce the energy consumption, a suspended membrane of silicon rich silicon nitride SiNx makes it possible to carry out the heat insulation between the heater and the substrate. Platinum (Pt) thin film (3000 A) with titanium (300 A) adhesion layer on SiNx/Si substrate is used for the hot resistor. Among the methods of Pt deposition tested, electron beam evaporation gives the best results for the temperature coefficient of resistance (TCR) of Pt. Its response time is about 6 ms. Sensitivity in laminar and turbulent flow range are respectively 4.80 mV/(m/s)0.45/mW and of 0.705 mV/(m/s)0.8/mW for about 20 mW power supplied. The experiments show that the temperature rise of the sensor is not sensitive to the ambient temperature. Moreover, sensor response shows no significant changes according to parallel or perpendicular orientation of the gas flow.

Growth of Bi2Te3 and Sb2Te3 thin films by MOCVD

Abstract Metal organic chemical vapor deposition (MOCVD) has been investigated for elaboration of Bi 2 Te 3 and Sb 2 Te 3 using TMBi (Trimethylbismuth), TESb (Triethylantimony) and DETe (Diethyltellurium) as metal–organic sources. Their thermoelectric and physical properties were studied versus growth conditions. The MOCVD elaboration of Bi 2 Te 3 and Sb 2 Te 3 was carried out in an horizontal reactor for a temperature varying from 400 to 500°C, a total hydrogen flow rate D T varying from 3 to 6 l mm −1 and ( R VI/V ) ratio ranging from 1.5 to 15. The thin films were deposited on pyrex and silicon substrates. The partial pressure of the V element varied between 0.5 10 −4 to 2 10 −4 atm to obtain high growth rate for micro-peltier applications. The cristallinity was investigated by X-ray diffraction and we observed a typical preferential c -orientation. The SEM micrographs show the layers quality and confirms the hexagonal structure. The microprobe data indicate that the stoichiometry of Bi 2 Te 3 and Sb 2 Te 3 is constant for all thickness of the epitaxial films (0.3–7 μm). The films are always n-type conduction for Bi 2 Te 3 and p-type for Sb 2 Te 3 . Seebeck coefficient and the minimum values of the resistivity were found close to −210 and +110 μV K −1 , 9 and 3.5 μΩ.m for Bi 2 Te 3 and Sb 2 Te 3 , respectively. Electrical measurements (mobility and carrier density) were performed by Van der Pauw method. For the two materials, the best values of thermoelectrical properties were obtained at a growth temperature closed to 450°C and a VI/V ratio varying from 2 to 8. The thermoelectric properties of the two materials stay constant when the growth rate is increasing to value higher than 1.5 μm h −1 . This result is very interesting for thick film applications. The previous objective of these experimental results has been to perform the thermoelectric properties of n- and p-type films by establishing first suitable deposition conditions and the elaboration of ternary alloys is now possible.

Porous silicon layers used for gas sensor applications

Abstract In this communication we report on the elaboration of porous silicon layers for gas sensor applications. We describe our test system for gas sensors, and we investigate the electric characteristics of porous silicon layers (p type) under different gases and levels of humidity.

Porous silicon layer coupled with thermoelectric cooler: a humidity sensor

Abstract In this work, an original humidity sensor is described. It is based on the study of the capacitance variation of a porous silicon layer (PSL) during water condensation induced by a commercial small-size thermoelectric cooler (TEC). The measurement principle is to detect the weak increase of capacitance created when water condensation occurs in a PSL stuck on a TEC. This important variation of capacitance is related to the high difference between the dielectric constant of PS ( e r e r ≅80). The dielectric constant of PS ranges from these of silicon oxide ( e r =3.9) to these of silicon ( e r =12) [H. Mathieu, Physique des semiconducteurs et des composants electroniques, Masson, 1987, p. 36]. Experimental measurements are performed in a climatic chamber for several values of relative humidity from 10% to 95% and for a TEC current equal to 0.43 A for the cooling part of the process. The analysis of the PS capacitance leads to information over the condensation formation during the TEC cooling. A quick increase of the capacitance appears after a delay time, τ , of 0.5–2 s from the start of the TEC cooling. The higher the humidity level, the faster the capacitance increase. It is possible to draw the capacitance reached after 1 s, from the start of the TEC cooling as a function of the relative humidity level.

Elaboration of Bi2Te3 by metal organic chemical vapor deposition

Abstract Bi2Te3 layers were elaborated for the first time using metal organic chemical vapor deposition. The films composition is stoichiometric when the following conditions are verified: substrate temperature lower than 500 °C, VI/V ratio greater than 3, TMBi partial pressure lower than 2 × 10−4 atm. By X-ray diffraction and MEB observation, we noticed the polycrystalline structure of the layers. The high thermoelectric power (+ 190 V K−1 for the n-type layer and −94 V K−1 for the n-type layer) of this material is promising for device applications.

Acoustic investigation of porous silicon layers

Porous silicon (PS) layers are formed on p+ -type silicon wafers by electrochemical anodization in hydrofluoric acid solutions. Microechography and acoustic signature, V(z), have been performed at 1.5 GHz and 600 MHz, respectively, in order to study the elastic properties of PS layers. The thicknesses of PS layers were measured and longitudinal, shear and Rayleigh velocities and Youngs modulus were obtained as a function of porosity. Equations showing the porosity dependence of bulk wave velocities and Youngs modulus have also been proposed.

Realization of porous silicon membranes for gas sensor applications

Abstract In this paper we report on the elaboration of porous silicon membranes for gas sensor applications. We attempted to replace the Teflon membrane of an oxygen sensor by a porous silicon membrane. The sensor was originally designed to measure the concentration of oxygen dissolved in aqueous solutions and had to be slightly modified. Diffusion times in the range of 10 min have been found.

Porous silicon membranes for gas-sensor applications

Abstract A porous silicon membrane (PSM) for an electrochemical oxygen microsensor has been achieved. Both p+- and n+-type materials have been studied. On the p+-type silicon, the porosity is found to be homogeneous through the complete thickness of the membrane, while n+-type membranes appear less homogeneous. The permeability of the PSM to oxygen has been tested with the help of a classical sensor. The diffusion process is found to be much more efficient for n+-type than for p+-type membranes. For PSM thicknesses ranging from 150 to 270 μm, the diffusion time varies from 7 to 10 min.

Acoustic microscopy investigation of porous silicon

Abstract Porous silicon (PS) layers were obtained from (100) p+-type silicon substrate of thickness 290 μm with a resistivity of (1–8) × 10 −2 Ω cm and porosity values ranging from 20% to 55%. Microacoustic techniques were performed to investigate the elastic properties of the PS layers. The porosity dependence of the longitudinal and Rayleigh velocities of the PS layers was obtained by microechography and acoustic signature V(z) respectively. Moreover, acoustic imaging was performed in order to analyse qualitatively the homogeneity of the samples. The profile of the PS layer-non-porous silicon interface was also obtained by time-resolved measurements.

X-ray diffraction studies of electrostatic sprayed SnO2: F films

Fluorine-doped tin oxide films were deposited by electrostatic spray pyrolysis technique (ESP), on 1cmx1cm Corning 7059 substrates. The structural and electrical properties of the deposited films with different doping levels are studied. Relative variations in the structural properties were explained on the basis of structural factor calculations. The results show that the incorporation of fluorine atoms took place only at substitutional sites leading to an increase in free carrier concentration.